------hdb3.vhd  HDB3 coder in VHDL------
--Desinger: Long Yachun
--Date:     2004-12-10

library ieee;
use ieee.std_logic_1164.all;

entity hdb3 is
port(codein: in std_logic;  
     clk   : in std_logic;
     clr   : in std_logic;               --復位信號
     codeout: out std_logic_vector(1 downto 0));
end hdb3;

architecture rtl of hdb3 is
signal count0  : integer:=0;
signal codeoutv: std_logic_vector(1 downto 0);
signal s0      : std_logic_vector(4 downto 0):="00000";	
signal s1      : std_logic_vector(4 downto 0):="00000";	
signal clkb    : std_logic;
signal count1  : integer range 1 downto 0;
signal codeoutb: std_logic_vector(1 downto 0);
signal flagv   : integer range 1 downto 0;
signal firstv  : integer range 0 to 1;
signal flag1b  : integer range 1 downto 0;

component dff                            --調用元件dff
  port (d   : in std_logic;
        clk : in std_logic;
        q   : out std_logic);
end component;

begin
  add_v:process(clk,clr)                 --插符號V進程
        begin
          if(rising_edge(clk)) then
            if(clr='1') then
              codeoutv<="00";            --用00代表0
              count0<=0;
            else 
              case codein is 
                when '1'=>
                    codeoutv<="01";      --用01代表1
                    count0<=0;
                when '0'=>
                    if(count0=3)then     --當有4個連續的0時插入V
                        codeoutv<="11";  --用11代表V
                        count0<=0;
                    else
                        count0<=count0+1;
                        codeoutv<="00";
                    end if;
                when others=>
                    codeoutv<="00";
                    count0<=count0;
              end case;
             end if;
           end if;
  end process add_v;
  
  s0(0)<=codeoutv(0);
  s1(0)<=codeoutv(1);  
  ds11: dff port map(s1(0),clk,s1(1));   --調用庫中的D觸發器來實現延遲作用
  ds01: dff port map(s0(0),clk,s0(1));
  ds12: dff port map(s1(1),clk,s1(2));
  ds02: dff port map(s0(1),clk,s0(2));
  ds13: dff port map(s1(2),clk,s1(3));
  ds03: dff port map(s0(2),clk,s0(3));
  bclk: clkb<=not clk;

  add_b:process(clkb)                    --插入符號B進程
      begin
        if(rising_edge(clkb)) then
          if(codeoutv="11") then
            if(firstv=0) then
              count1<=0;
              firstv<=1;
              s1(4)<=s1(3);
              s0(4)<=s0(3);
            else
              if(count1=0) then          --用10代表B
                  s1(4)<='1';
                  s0(4)<='0';
                  count1<=0;
              else
                  s1(4)<=s1(3);
                  s0(4)<=s0(3);
                  count1<=0;
              end if;
            end if;
          elsif(codeoutv="01") then
            count1<=count1+1;
            s1(4)<=s1(3);
            s0(4)<=s0(3);
          else
            s1(4)<=s1(3);
            s0(4)<=s0(3);
            count1<=count1;
          end if;
        end if;
  end process add_b;
  codeoutb<=s1(4)&s0(4);

  output:process(clk)                    --單極性變雙極性的進程
      begin
        if(rising_edge(clk)) then
          if((codeoutb="01")or(codeoutb="10"))then   --1 or B
            if(flag1b=1) then
              codeout<="01";
              flag1b<=0;
            else
              codeout<="11";
              flag1b<=1;
            end if;
          elsif(codeoutb="11") then      ---V
            if(flag1b=1) then
              codeout<="11";
            else 
              codeout<="01";
            end if;
          else 
            codeout<="00";
            flag1b<=flag1b;
          end if;
        end if;
  end process output;

  end rtl;